Production of direct bonded basic brick



United States Datent O 3,301,690 PRODUCTION OF DIRECT BONDED BASIC BRICKGeorge R. Rigby, Lachute, Quebec, Canada, assiguor to CanadianRefractories Limited, Montreal, Quebec, Canada No Drawing. Filed Aug.27, 1964, Ser. No. 392,617 Claims priority, application Great Britain,Aug. 29, 1963, 34,302/ 63 1 Claim. (Cl. 106-59) This invention relatesto the manufacture of direct bonded basic brick.

A typical basic'refractory brick for use, for instance, in open hearthroof construction, comprises 80% graded magnesia (MgO) and 20% 4 +10mesh (Tyler standard) chrome ore.

Normally, basic brick, containing most or all of the chrome ore fractionas coarse grains have, after firing, a gap around the chrome grainswhich, therefore, fit loosely into the brick matrix. This gap isproduced by the partial dissolution of the chrome spinel constituents inthe surrounding magnesia matrix during initial firing. Furthermore, thesilicate gangue associated with the chrome ore melts and thendistributes itself around the chrome grains as a silicate halo or fringeon the matrix side of the gap.

It is well known that open hearth roof brick absorb both iron oxide (FeO and lime (CaO) at the hot face in service and these migrate back intothe brick, the lime silicates migrating farther behind the hot face thanthe Fe O where they freeze and concentrate inzones. With a conventionalbrick containing both a gap and silicate halo around the chrome grains,there is a mechanism of failure in which, firstly, the silicate halomelts and migrates after combination with either the absorbed Fe O orCaO and, since this -is already associated with a gap, the space betweenthe chrome grains and matrix is further widened. This eventually createsa series of channels through the brick along which Fe O and silicatescan migrate readily. As they migrate, MgO is dissolved making thesechannels larger until they take the form of conspicuous voids located ashort distance behind the hot face. The next stage is for these voids tobe linked together by means of horizontal cracks, and the final stage isfor the face of the brick to drop off as a peel which is so oftenobserved in basic brick that have absorbed molten slags. It is,therefore, desirable, in order to increase service life, that thesilicates in the brick should be kept at a minimum and should bedistributed throughout the brick and not concentrated in the mostvulnerable areas around the chrome grains. Moreover, it is desirablethat any gaps around the chrome grains should be dispersed so that nopreferential channels are established as a result of absorption andmigration of liquid slag.

The invention resides in a magnesite chrome refractory brick comprising-40% particles of chrome ore containing not more than about 5% SiO andhaving a particle size of 4 +28 mesh, granular magnesia containing notmore than 1% Fe O 2% CaO, and 2% SiO and having a particle size of 4,and 540% particles of a clinker having a particle size of 65 mesh andbeing a fired homogeneous mixture of magnesia and 5 to 15% of a crystalgrowth promoting agent selected from the group consisting of chrome ore,iron oxide, titanium dioxide, and chromium oxide.

The invention also resides in a method of making magnesite chromerefractory brick which comprises forming a clinker by firing to atemperature to form a homogeneous solution, a mix consisting essentiallyof magnesia and 5 to 15% of a material selected from the groupconsisting of chrome ore, iron oxide, titanium dioxide, and

3,301,690 Patented Jan. 31, 1967 ice 2% CaO and 2% SiO and having aparticle size of portant to have them present only in the fines.

4 +65 mesh, and 10 to 40% of said clinker particles, pressing saidmixture into shapes, and firing said shapes at a temperature of about1600-1700 C.

In accordance with the invention, most or all of the fines (65 mesh)portion of the conventional brick is replaced with a special clinkermade by firing at least to a temperature to obtain chemical homogeneity,a mixture of magnesia and 5% to 15% of a crystal growth promoting agent.The clinker is cooled and ground to at least 65 mesh (Tyler standard).

Crystal growth promoting agents of the type employed are partiallysoluble in magnesia, so that the special clinker is a homogeneous solidsolution at high temperatures, although at room temperatures there maybe a microscopically disseminated precipitate of a separate phase with aspinel structure.

When a brick is made, the fines of the brick mix fill the intersticesaround the coarse and intermediate grains. Thus, each coarse chromegrain is surrounded by the fine clinker which, upon firing,recrystallizes to form larger magnesia crystals. This crystal growthdisrupts the edges of the MgO grains, detaching individual periclasecrystals, which, because of the expansion and loosening of the MgO graintexture, move across the gap and attach themselves to the surface of thechrome grains.

Direct bonding, therefore, is assisted by promoting the solution ofspinels into the MgO which, in turn, is promoted by high firingtemperatures.

While the agent employed in the pre-sintering step may be iron oxide (FeO chromium oxide (Cr O titanium dioxide (TiO or fine chrome ore, themost suitable ones are Fe O and finely divided chrome ore. About 5% to15 by weight of such agent is used in forming the special clinker.

to disappear and the discontinuous structure, and, therefore, spallingresistance, is lost.

The agents used in preparing the special clinker act so effectively topromote grain growth, that it is im- The coarse and intermediate size ofmagnesia grains should be of high purity, low particularly in theoxides, of chrominum, titanium, iron, and silicon. They will then retaintheir identity throughout the service life of the brick, at least untilthe iron oxide pick-up reaches them.

- As previously indicated, it is desirable that the silica (SiO contentof the brick be kept at a minimum. Since magnesias with very low silicacontents are readily available nowadays, the SiO is largely contributedby the chrome ore. Thus, it is desirable to utilize chrome orecontaining not more than 5% and preferably not more than 2% and tomaintain the chrome ore content at the lowest effective level. Such SiOas remains should be dispersed as widely as possible throughout thebrick and this is attained by high firing temperatures which cause thesilicates to migrate away from the chrome grains further into themagnesia matrix so that the silicate halo is dispersed. This is a secondreason why high firing temperatures are desirable. The chrome ore isgraded 4 +10 mesh (Tyler standard). Coarse chrome ore is consideredessential since this is the constituent of the brick which retains itsidentity longest and maintains spalling resistance during service.

The magnesia used in the mix should contain not more than 1% Fe O 2%CaO, and 2% SiO and has a particle size of 4 +65 mesh (Tyler standard).

The brick mix comprises 10% to 40% of chrome ore, magnesia, and 10% to40% clinker, of the specified characteristics. It will be apparent thatthe size grading of the brick mix is about the same as used inconventional brick making; that is, using about 30-40% of 65 meshmaterial.

Normally a binder, such as waste sulfite liquor of Epsom salts, is usedin the mix primarily to impart sulficient strength to the brick shapesto permit handling of the same between the pressing and firingoperations. Such binders usually either disappear during firing or leavesuch little residuum as to have no effect on the behaviour andproperties of the brick.

The brick is pressed in a conventional refractory brick making press andfired at a temperature of 1600 to 1700 C., and preferably about 1650 C.

While not intended to he limitative, the following procedure may befollowed in forming the mix:

The coarse chrome ore is initially charged into the mixer and thensufficient binder to coat it. Next, the special clinker powder is addedin sufficient quantity to coat the chrome ore grains. Finally, thecoarse magnesia, the rest of the binder, and fine magnesia are added. Inthis instance, only a portion of the fines comprises the special clinkerpowder. Thus, by following this procedure, not only is the specialclinker powder placed adjacent the coarse chrome grains only, but thequantity required of such powder is lessened.

The brick produced in accordance with the invention as compared withconventional brick, has an increased modulus of rupture, and anincreased modulus of elasticity, both at room and elevated temperatures.

The following examples are illustrative of the invention. The magnesiaemployed was the same in all instances and contained not more than 1% FeO 2% CaO, and 2% SiO The chrome ore employed was also the same in allinstances and contained not more than 5% SiO Example I A clinker wasmade from Fe O and 90% magnesia. After cooling, it was ground to atleast 65 mesh.

A brick mix was made containing:

Percent -10 +28 mesh chrome ore 20 4 +65 mesh magnesia 40 65 meshclinker 40 A small amount of binder (not more than 5% of the total mix)and consisting of waste sulfite liquor was employed as the binding agentfor the shapes before firing.

The mix was pressed into shapes and fired in conventional manner at atemperature of about 1650 C.

Example II This example is the same as Example I except that the clinkerwas made from 10% chrome ore and 90% magnesia.

Example III This example was conducted for comparison purposes andfollowed the conditions of the other examples except that the brick mixwas conventional, as follows:

Percent 10 +28 mesh chrome ore 20 4 +65 mesh magnesia 40 65 meshmagnesia 40 Thin sections of brick made by each of the examples weremeasured petrologically for direct bonding characteristics. Each of thesections from Example I and II showed well developed direct bonding inthat the periclase crystals were attached to the chrome grains along theborder thereof. The sections of brick made by Example III showed onlyslight attachment of magnesia crystals to some chrome grains with mostof the chrome grains having gaps around them.

A modulus of rupture test at a temperature of 1250 C., which issometimes employed as an indication of direct bonding development, hadthe following results:

Example I-3l0 p.s.i. Example II310 p.s.i. Example III- p.s.i.

These tests were made on one inch square bars. It will be apparent thathigher but still relative values would be obtained when testing bricksof full size.

A cold modulus of rupture test had the following results:

Example I440 p.s.i. Example 11-450 p.s.i. Example III410 p.s.i.

The screen sizes given throughout this specification and accompanyingclaim are Tyler standard.

I claim:

A method of making magnesite chrome refractory brick which comprises thesuccessive steps of mixing a binder with a quantity of particles ofchrome ore containing not more than 5% SiO and having a particle size of4 +28 mesh to coat said particles with said binder, mixing with saidbinder-coated particles a quantity of particles of clinker having aparticle size of 65 mesh to coat said binder-coated particles with saidclinker particles, said clinker having been formed by firing a mixtureconsisting essentially of magnesite and 5% to 15% of a material selectedfrom the group consisting of chrome ore, iron oxide, titanium dioxide,and chromium oxide at a temperature to obtain chemical homogeneitythereof, forming a mixture consisting essentially of said clinker-coatedparticles and an additional quantity of binder and a quantity ofmagnesia particles containing not more than 1% Fe O 2% CaO, and 2% SiOand having a particle size of 4 +65 mesh, said mixture containing 540%of said chrome ore particles, 540% of said clinker particles, and thebalance said magnesia particles, pressing said mixture into shapes, andfiring said shapes at a temperature of 16001700 C.

References Cited by the Examiner UNITED STATES PATENTS 4/1965 Davies etal. 10659 10/1965 Davies et a1. 106-59

